A rapid and low-cost handheld method for measuring the mean leaf inclination angle of crop canopies

Leaf inclination angle (LIA) strongly affects canopy photosynthesis and is an important trait for crop productivity. Difficulty in determining mean LIA throughout the canopy makes it a trait rarely evaluated in crop science research. Here, we report a novel, low-cost, simple method to rapidly measure the mean LIA throughout the canopy. We captured RGB and depth images of the crop canopy with a compact stereo depth camera aligned parallel to the ground in a handheld manner. The mean LIA was estimated through simple vector calculations. To validate the accuracy of this method, we used tests with a white board fixed at known angles and single leaves whose angle had been measured with a digital level. The mean LIA was strongly correlated with the mean tilt angle-a trait similar to LIA that was measured with a plant canopy analyzer, an expensive instrument for measuring leaf area index. Using our method, we were able to detect differences in LIA among cultivars, growth stages, and plant densities in rice, barley, soybean, and wheat. Because this method is very easy and has minimal financial, time and labor requirements, it is likely to be highly beneficial for crop scientists whose primary research is conducted in fields.